1.1 Field of the Invention
This invention relates to the field of well logging and, more particularly, to an improved method and apparatus for making focused downhole measurements of formation characteristics. The invention has general application in the well logging art, but is particularly useful in logging-while-drilling.
1.2 Description of Related Art
Resistivity logging is a well-known form of electromagnetic ("EM") propagation logging. Resistivity logging is used for measuring and evaluating the characteristics of potential hydrocarbon bearing zones in subsurface formations. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while low resistivity formations are generally water saturated.
In conventional logging techniques, a number of antennae or coils are mounted on a well tool. The tool is lowered into a borehole on the end of a cable, or wireline. An alternating current energizes one or more transmitter coils to emit EM energy into the formations. The emitted energy propagates through the formations or induces currents in the formations surrounding the borehole. The EM energy or currents are detected and measured by one or more receiver coils on the tool. The cable, which is attached to some sort of mobile processing center at the surface, is the means by which measured data is sent up to the surface. With this type of wireline logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is being pulled uphole.
An alternative to wireline logging techniques is the collection of data on downhole conditions during the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize performance. Formation information collected during drilling also tends to be less affected by fluid invasion processes or other undesirable influences as a result of borehole penetration, and therefore are closer to the properties of the virgin formation.
Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as measurement-while-drilling ("MWD") techniques. Similar techniques focusing more on measurement of formation characteristics than on movement of the drilling assembly are know as logging-while-drilling ("LWD"). However, the terms MWD and LWD are often used interchangeably, and the use of either term in the present disclosure should be understood to include both the collection of formation and borehole information, as well as data on movement of the drilling assembly,
U.S. Pat. No. 3,551,797 describes a conventional EM propagation logging technique. The '797 patent describes the transmission of EM energy into the formations, where energy shed back into the borehole is measured by receivers to determine the relative attenuation and/or the phase shift of the EM energy propagating in the formation. See also B. Clark et al., Electromagnetic Propagation Logging While Drilling: Theory and Experiment, SPE SIXTY-THIRD ANNUAL TECHNICAL CONFERENCE AND EXHIBITION, paper 18117, 1988.
U.S. Pats. Nos. 4,899,112 and 5,594,343 (both assigned to the assignee of the present invention) disclose conventional well logging tools used to evaluate the resistivity of formations in LWD operations. The '112 patent concerns the determination of formation resistivity at different radial depths of investigation with the use of receivers placed between two transmitters. The '343 patent concerns the determination of formation properties at different radial depths of investigation with the use of multiple transmitters at various spacings from a pair of receivers.
If the antenna coils on these tools were perfectly configured and balanced in a theoretically ideal system, the EM energy emitted by the coils would propagate in a mode known as a transverse electric ("TE") mode, of the type generated by an ideal vertical magnetic dipole. However, under actual operating conditions, there are various factors that give rise to the generation of significant undesired EM field components. One approach to alleviating this problem is with the use of antenna shields to reduce the transmission and/or reception of spurious and unwanted EM field components. These shields are typically used in conjunction with each coil on the tool.
U.S. Pats. Nos. 4,536,714 and 4,949,045 (both assigned to the assignee of the present disclosure) disclose conventional antenna shields employed in these tools to provide mechanical protection for the coils and to guarantee the passage of desired EM field components. As shown in FIG. 1a, these shields 10 are in the form of a metal cylinder that has slots 12 in the axial direction. The slot 12 pattern allows the azimuthal electric field (E.phi.) component of the EM energy to pass, but prevents radial (Er) and axial (Ez) electric field components from passing through the shield, either from within (in the case of a transmitter) or from without (in the case of a receiver). An alternative viewpoint is to represent each axial slot 12 as an axial magnetic dipole, as shown in FIG. 1b. These magnetic dipoles are sensitive to axial magnetic fields (Bz), but they are not sensitive to azimuthal magnetic (B.phi.) fields. The shielded coils are thus rendered insensitive to parasitic transverse magnetic ("TM") EM fields associated with borehole is modes, and which have radial (Er) and axial (Ez) electric fields and azimuthal magnetic fields (B.phi.).
An emerging technique in the field of well logging is the use of tools incorporating tilted antennae, i.e., where the coils are tilted with respect to the tool axis. These apparatus are configured as such in an effort to alter the direction of the downhole measurement. U.S. Pat. No. 5,508,616 describes an induction tool incorporating tilted transmitter and receiver coils. PCT Application WO 98/00733, Bear et al., describes a logging tool including triaxial transmitter and receiver coils. U.S. Pat. No. 4,319,191 describes a logging tool incorporating transversely aligned transmitter and receiver coils. U.S. Pat. No. 5,115,198 describes a tool including a triaxial receiver coil for measuring formation properties. U.S. Pat. No. 5,757,191 describes a method and system for detecting formation properties with a tool including triaxial coils. Common to these apparatus and techniques, however, is the need to manipulate the antenna itself in order to achieve any directionality of measurement.
It is desirable to obtain an apparatus that can provide directional downhole measurements without being limited to the use of tilted coils. Further, it is desired to implement a system that can provide an azimuthally focused downhole measurement with the use of tilted or non-tilted coils.